In Portugal, from 1930 to 1955, over 2400 persons received Thorotrast - a colloidal suspension of thorium dioxide, used as a radiological contrast medium mainly for the visualization of blood vessels. Once the drug is injected systemically, it remains forever in the body, practically with no elimination, and it accumulates in the liver, spleen and tributary lymph nodes. It is not surprising that pathological situations may develop, caused by the presence of the thorium dioxide, which is a radioactive substance. The purpose of this research is to trace the greatest number of such patients (of which more than 1900 received the drug by a systemic route) and learn the possible undesirable effects of that contrast drug. Clinical examens, radiographies, several laboratorial tests (including hematological studies as myelograms, chromosome aberration studies, search for alpha-feto-protein, etc.), and pathological studies (optic and electron microscopy) of material coming from biopsies or necropsies, are done whenever possible. We have been able so far to trace about 50% of the patients who received the drug, and to follow them up until the present moment or until death. At the moment there are still 311 Thorotrast patients alive. The more frequent pathological situations we have found that can be attributed to the drug are: liver tumors (mainly hemangioendotheliomas and cholangiocarcinomas), fatal blood dyscrasias (mainly acute leukaemias), liver fibrosis and local granulomata. In our opinion all these pathological consequences are due, with great probability, to the radioactivity of Thorotrast. The Thorotrast patients constitute, thus, a unique population submited to a continuous source of radiation, and extrapolations can be done from our results to other radiation emitters. Since 1966 we have been following also a group of control cases - patients injected with a non-radioactive contrast drug; we were able so far to trace 812 of such cases. The pathological consequences attributable to the contrast drugs are non-existent compared with the Thorotrast ones.